#!/usr/bin/env python3
# coding=utf-8

import time, threading

# 假定银行存款
balance = 0
lock = threading.Lock()


def change_it(n):
    # 先存款后取，结果应该为0
    global balance
    balance = balance + n
    balance = balance - n


def run_thread(n):
    for i in range(1000000):
        # 先要获取锁
        lock.acquire()
        try:
            change_it(n)
        finally:
            # 改完一条释放锁
            lock.release()
        change_it(n)


t1 = threading.Thread(target=run_thread, args=(5,))
t2 = threading.Thread(target=run_thread, args=(8,))
t1.start()
t2.start()
t1.join()
t2.join()
print(balance)
